SpaceX and NASA have pushed the first test flight of the Crew Dragon capsule back by 10 days, partly to avoid a traffic jam at the International Space Station (ISS).

The maiden orbital mission of SpaceX's astronaut taxi — an uncrewed flight to the ISS known as Demo-1 — is now scheduled to launch on Jan. 17, NASA officials announced on Friday (Dec. 7). That's four days after one of Crew Dragon's robotic, cargo-carrying cousins departs the orbiting lab.

If all goes according to plan with Demo-1, Crew Dragon will berth with the ISS, stay attached to the orbiting lab for a few weeks, and then come back down to Earth in a parachute-aided ocean splashdown, as its cargo cousins do.

Demo-1 will help prove out all these activities before Crew Dragon carries astronauts on the Demo-2 mission, a flight to the ISS that's currently targeted for June 2019. Between those two missions, SpaceX will conduct an uncrewed "in-flight abort" test with the capsule, demonstrating its ability to get astronauts away from danger in the event of a launch anomaly.

"We still have more work to do as the certification process, hardware development and readiness reviews continue," Kathy Lueders, manager of NASA’s Commercial Crew Program, said in a statement.

"The key readiness reviews along with NASA’s continued analysis of hardware and software testing and certification data must be closed out prior to launch," she added. "The upcoming steps before the test missions are critical, and their importance can't be understated. We are not driven by dates, but by data. Ultimately, we'll fly SpaceX Demo-1 at the right time, so we get the right data back to support the in-flight abort test and the next test flight when our astronauts are aboard."

SpaceX holds a multibillion-dollar commercial-crew contract with NASA, as does aerospace giant Boeing. Boeing is developing a capsule called the CST-100 Starliner, which is scheduled to fly its first uncrewed test mission in the spring of 2019.

Once Crew Dragon and Starliner are up and running, NASA will no longer be entirely dependent on Russian Soyuz rockets and spacecraft to get American astronauts to and from the ISS. This has been the situation since July 2011, when NASA grounded its space shuttle fleet.

SpaceX also holds a cargo contract with NASA and has flown 16 resupply missions to the orbiting lab since 2010. Northrop Grumman makes ISS cargo flights as well, using Antares rockets and Cygnus freighters.